Acta Crystallographica Section B

Structural Science

Volume 57, Part 3 (June 2001)

research papers

Acta Cryst. (2001). B57, 231-236 [ doi:10.1107/S0108768100020322 ]

Structural basis for the phase transitions of Cs₂HgCl₄

B. Bagautdinov, A. Jobst, J. Ludecke and S. van Smaalen

Abstract: The a₀ × b₀ × 2c₀ twofold superstructure of dicaesium mercury tetrachloride, Cs₂HgCl₄, at T = 120 K has been determined by single-crystal X-ray diffraction using synchrotron radiation. Lattice parameters were found as a = 9.7105 (2), b = 7.4691 (1), c = 26.8992 (4) Å, and [beta] = 90.368 (1)° with the supercell space group P2₁/c. Refinements on 1828 observed unique reflections converged to R = 0.053 (wR = 0.057) using anisotropic temperature factors for all atoms. This phase is the stable phase of Cs₂HgCl₄ below 163 K. A quantitative comparison is made of the distortions of the 2c₀ superstructure with the undistorted phase that is stable at room temperature, and with the 3c₀ and 5a₀ superstructures that are stable at temperatures between 163 K and room temperature. The principal difference between the 2c₀ superstructure and all other phases of Cs₂HgCl₄ is that the Cs cations are displaced away from the centers of their coordination polyhedra in the 2c₀ superstructure. The structural basis for the driving force of the series of phase transitions in this compound is found in the variations of the environments of Cs atoms and in the variations of the distortions of the HgCl₄ tetrahedra.

Keywords: phase transitions; low temperature; superstructure.